Bullets for hunting firearms are conventionally divided between full-caliber bullets and subcaliber bullets.
Conventional full-caliber bullets are generally made of lead or a lead alloy. They do not fragment and expand only very slightly upon impact with the target. Certain bullets include fracture initiators which cause the body of the bullet to fragment upon impact. These bullets are fired at relatively low velocity, less than 500 m/s, and are ineffective against moderately or highly resistant targets, such as large game animals, for example wild boar.
Subcaliber bullets include a subprojectile, possibly fin-stabilized, combined with a full-caliber sabot, which separates upon leaving the barrel of the gun.
All these conventional bullets have drawbacks generally associated with their inaccuracy, with their ballistic dispersion and their lack of effectiveness against targets having hard parts.
So-called expanding bullets, which deform upon impact by mushrooming, including against moderately resistant targets, have been described in the art. For example, U.S. Pat. No. 4,685,397 describes a bullet with an ogival body, the head of which includes a blind hole closed off by an insert capable of penetrating the hole upon impact and of pushing the wall of the ogive outward. Patent EP-A-918 208 describes another embodiment of such a bullet having a cylindrical body and an ogival head, with a blind hole along its axis, closed off by an insert, and supplemented with a peripheral annular groove. Thus, upon impact, the insert penetrates the blind hole and causes the ogival head to deform by expansion, which is facilitated by the annular groove. Application PCT WO 03/093758 describes a partially fragmenting expanding bullet that includes an orifice located in the ogival head, completely closed by a plug fitted onto a rod, the movement of which causes the bullet body to deform.
However, expanding ammunition of this type have the drawback of random control of the deformation according to the shooting conditions, being accompanied by a risk of the ogival head of the bullet fragmenting.
Further improved ammunition for hunting guns have been developed on the principle of the “dart” bullet. Ammunition of this type are described for example in FR-A-2 555 728. Upon impact of ammunition of this type on the target, the dart deforms, and it is desirable that this deformation be controlled and not cause excessive dislocation of the dart into several small fragments which could prove to be dangerous. Patent FR-A-2 795 170 describes ammunition that meet this objective, which essentially consist of a full-caliber or subcaliber bullet comprising a profiled front part, a central part and a rear part, which is possibly finned, and including, lying along its axis, a supported internal dart which is at least as rigid as the bullet body. Bullets of this type are very accurate and have the advantage of retaining their mass upon impact against the target. According to this art, the diameter of the ogival nose represents between 40% and 50% of the maximum diameter of the bullet, thereby giving it a high aerodynamic drag. These bullets are therefore mainly intended for what is called “brush” shooting, over short and medium distances, of less than 150 m in the case of rifles and around 50 to 60 m in the case of shotguns.
Another type of ammunition in which the bullet body includes an orifice completely closed by a closure cap is described in U.S. Pat. No. 6,349,651, but the closure cap bears on the front face of the bullet body. An embodiment of a hunting bullet with delayed expansion is described in patent EP 1 394 498, in which the bullet body is completely penetrated by an element which, upon impact, causes the bullet body to deform. Patent DE 19 903 395 describes a bullet having a metal core with an enlarged base, supporting an outer jacket with an open ogival head protruding from the central core.
However, the ballistic performance of the ammunition of this type is inferior when they are fired from hunting guns in which the initial velocity is relatively slow, that is to say less than about 600 m/s. In addition, their relative high aerodynamic drag has the effect of limiting the velocity of impact on the target. It is known that the velocity of impact on the target, the mass of the bullet and its configuration are three essential parameters that must be controlled in order to achieve a good stopping power by a cavitation effect in the target.
Thus, at the present time there is a need to be able to have ammunition, especially for hunting guns, which exhibit controlled expansion upon impact, in particular on weakly resistant targets, and which release their energy effectively.